1. Field of the Invention
The present invention relates to an apparatus for depositing a thin film by chemical vapor deposition (hereinafter referred to as CVD), and more particularly, to an apparatus for depositing a thin film while heating a substrate on which the film is to be deposited. More specifically, the present invention relates to a thin-film depositing apparatus adapted for the selective CVD process.
2. Description of the Prior Art
The CVD process is one of the thin-film depositing methods that are used for the manufacture of semiconductor integrated circuits. This process is used, for example, to deposit a thin film of polycrystalline silicon from silane (SiH.sub.4) as a material gas, or a thin film of silicon dioxide from organosilane. Recently, the CVD process has been tried as a method for depositing an aluminum film from organoaluminum compound, or thin films of refractory metal and/or silicide thereof from halides of refractory metal.
The CVD process is effected by two methods; an atmospheric-pressure CVD method, in which a thin film is deposited under atmospheric pressure, and a low-pressure CVD method (hereinafter referred to as LPCVD) in which a thin film is deposited under reduced pressure. Presently, the LPCVD is preferred, since it is higher in productivity, and ensures a higher uniformity of film quality and a better step-coverage.
A batch system has conventionally been used in the thin-film deposition by the CVD process. According to this system, a thin film is deposited on a number of substrates at a time. If the surface of each substrate has a micro-structure, however, the batch system is liable to cause variations in the thickness of film on the individual substrates, sometimes entailing insufficient coverage. Thereupon, a "one wafer type" system has been proposed to counter such an awkward situation. According to this system, substrates are fed one by one into a reaction chamber for thin-film formation. In a CVD apparatus based on the "one wafer type" system, however, the thin-film deposition is performed while heating each substrate placed on a susceptor, in order to accelerate a filming reaction on the substrate surface. This entails the following problems.
In the first place, the susceptor, supporting the substrate, and that portion of a reaction container situated close to the substrate are heated to high temperature, so that a thin film is deposited not only on the surface of the substrate, but also on the surfaces of the heated portions of the apparatus. This indicates a waste of the material gas or the film deposition. Thus, costs are increased, and the rate of film deposition on the substrate surface is lowered.
Secondly, the thin film, deposited on the surface of the susceptor or the like in the aforesaid manner, grows with every cycle of filming operation. Ultimately, therefore, the film is separated from the susceptor surface, and adheres, in the deposit of dust, to the substrate surface during the thin-film deposition. Such adhesion of dust deteriorates the properties of the thin film on the substrate. In manufacturing semiconductor integrated circuits, this results in a fatal defect in the circuits, thus lowering the yield considerably. This problem often arises especially when the selective CVD process is used.
Thirdly, if a film is deposited on any other region than the substrate surface, a large amount of reaction byproduct arises in a reaction chamber. As a result, reactive gas in the vicinity of the substrate surface is complicated in composition, so that it becomes difficult to control the CVD process and the quality of the resulting thin film. This problem also arises frequently from the selective CVD. In the selective CVD process, moreover, its selectivity is deteriorated by the reaction by-product, so a film may be deposited on an undesired portion of the substrate surface.
The above problem is especially conspicuous when the LPCVD is used. The reason is that, in the LPCVD, the pressure between a substrate, on which a thin film is to be deposited, and a susceptor, carrying the substrate thereon, is reduced. Thus, the efficiency of thermal transfer between the susceptor and the substrate becomes so low that heat cannot be transferred easily from the susceptor to the substrate. As a consequence, the surface temperature of the susceptor becomes higher than that of the substrate, so that a film is liable to be formed on the susceptor surface.